In prior art, the structures of electrically pumped semiconductor lasers fall into two main categories: (1) "edge emitting"(or simply "edge") lasers and (2) "surface emitting" (or simply "surface") or vertical lasers. Both have an active region which typically comprises a planar pn junction. Typically the plane of this pn junction is parallel to a major surface of a semiconductor body, the major surface being considered somewhat arbitrarily to be horizontal. In an edge laser, light is emitted from one (or more) side edge(s) of the semiconductor, an optical cavity in the semiconductor being created in the semiconductor body by virtue of semi-reflecting optical mirror(s) located on the side edge(s). In a vertical laser, light is emitted from the top or the bottom (major) surface, or both, of the semiconductor, and optical cavity being created therein by virtue of semi-reflecting optical mirror(s) located on the top or bottom surface thereof, or both. Vertical lasers appear to be more attractive from the standpoint of not requiring the more difficult control over the side edge contours as is required in the case of horizontal lasers.
Typically, in a vertical laser each of the mirrors is formed by a quarter-wavelength-stack, such as a stack formed by a semiconductor superlattice. A vertical laser can be built as a double heterostructure, for example, by successive epitaxial growth of the follwing semiconductor layers in spatial sequence upon a semiconductor substrate: the bottom mirror, a bottom optical cladding region, the active region, a top cladding region, and the top mirror. A top electrode is then formed on the top major surface of the top mirror, for electrical access. Many such vertical lasers can be built simultaneously on a single such substrate in such a way that the amount of light--e.g., ON vs. OFF--emitted by each laser can be controlled by an electrical signal independently of all other lasers on the substrate. Thus vertical lasers appear especially attractive for use in practical applications where more than one independently controllable source of light is desired. Typically the amount of light emitted by each such vertical laser is determined by the electrical current injected into the laser through the top electrode.
In prior art, during operation a vertical laser suffers from undesirably large amounts of heat generation, whereby the efficiency and lifetime of the laser are undesirably low. Therefore it would be desirable to have a vertical laser in which the heat generation is reduced.